This study of synaptically connected CA3 pyramidal cells will attempt to answer the following questions: 1. Are pairs of CA3 pyramidal cells connected by a single synapse? 2. Is the distribution of synaptic responses skewed? 3. Does the size of the presynaptic bouton correlate with synaptic strength? 4. Does the size of the postsynaptic density correlate with the quantal size? 5. Does the number of docked synaptic vesicles correlate with the probability of release? These questions will be answered by recording synaptic responses between pairs of CA3 pyramidal cells while filling those cells with a cell impermeant tracer. The neuronal processes of the two cells will be reconstructed, and electron microscopy will be used to determine the ultrastructure of all synapses involved in the synaptic response. In pairs involving only a single synapse, the ultrastructure and synaptic responses will be compared. The results of this study will provide important information about the basic functional and structural properties of individual excitatory synapses in the hippocampus and will provide a solid foundation, that has been lacking, for future studies into the electrophysiological mechanisms and ultrastructural underpinnings of long-term potentiation.